This invention relates to an apparatus for driving a wire-dot print head in a wire-dot impact printer.
A wire-dot print head comprises a plurality of print wires, and a means for driving the print wires forward so that their ends impact on a sheet of paper. An inked ribbon is interposed between the ends of the print wires and the paper so that the impact of each wire causes the printing of a dot. Characters and graphic designs are printed as a matrix of dots by driving the print wires at appropriate times as the head travels across the paper.
In the well-known spring-release type of wire-dot print head, the means for driving each print wire comprises an armature, a plate spring, and an electromagnet. The plate spring is secured at one end. The print wire is attached to the armature, which is mounted on the free end of the plate spring. Normally a permanent magnet holds the spring in a flexed position in which the print wire is retracted. When the electromagnet of a print wire is energized (driven), it produces a magnetic field opposing the field generated by the permanent magnet. These two fields cancel each other, releasing the spring, which drives the print wire forward to print a dot. When the energizing current is removed from the electromagnet, the permanent magnet again attracts the spring, causing the print wire to return to its retracted position in preparation for printing the next dot.
A critical parameter in this driving means is the length of time for which the electromagnet is driven in order to print a dot. If this time is too short, the impact will be weak or absent, causing printing irregularities such as faint or skipped dots. If the driving time is too long, however, the print wire will be late in returning to its retracted position, so it will not be ready to print the next do unless the printing rate is reduced to an undesirably slow speed.
The optimum driving time depends on a plurality of factors, one of which is the voltage Vcc applied to the electromagnet. A prior-art scheme for controlling the driving time employs a resistor and capacitor connected in series between Vcc and ground, with the driving time regulated according to the charging time of the capacitor. This scheme automatically compensates for variations in Vcc.
This prior-art timing scheme, however, fails to compensate for factors such as the varying gap between the paper and the ends of the retracted wires (called the head gap), or for wire-to-wire variations in, for example, the strength of the spring, or for variations caused by magnetic interference inside the print head. To allow for such variations, it is necessary to add a margin to the driving time so that on the average the elecromagnet is driven for longer than the optimum time. As a result, the prior-art wire-dot print head driving apparatus is unnecessarily slow, consumes unnecessary current, generates unnecessary heat, and produces printed output of inferior quality due to the non-optimal driving time.